1. Field of the Invention
The present invention relates to an optical fiber production method and an optical fiber production apparatus, more particularly relates to an optical fiber production method which cools the optical fiber drawn from an optical fiber preform inside a heating furnace by a cooling gas by a cooling apparatus arranged beneath the heating furnace and then coats a resin on that optical fiber, and a production apparatus for the same.
2. Description of the Related Art
An optical fiber is formed by heating an optical fiber preform in a heating furnace and then drawing the same. For example, a single mode optical fiber is formed by a core having a diameter of 10 .mu.m at the center and a cladding having a diameter of 125 .mu.m the same. A coating of a protective resin is provided on the optical fiber drawn in the heating furnace on the surface circumference thereof by a resin coater positioned beneath the heating furnace.
The optical fiber immediately after it is drawn in the heating furnace and pulled out has a high temperature, for example, about 800.degree. C. When a protective resin is to be coated on an optical fiber which in such a high temperature state by a resin coater, the viscosity of the protective resin is increased by the heating, and therefore the protective resin cannot be coated well on the optical fiber. Therefore, the optical fiber has been force-cooled to a predetermined temperature after the optical fiber is drawn in the heating furnace and before the resin is coated on the optical fiber by the resin coater.
Japanese Unexamined Published Patent Application (Kokai) No. 53(1978)-125857 (hereinafter referred to as JPP 53(1978)-125857, same below) discloses a method of force-cooling the optical fiber by blowing air from a plurality of injection ports in a direction orthogonal to a running direction of the optical fiber. In this method, the cooling is insufficient if only one injection port is provided for cooling, therefore it is necessary to provide multiple injection ports for cooling. As a result, the cooling apparatus becomes complex, and consequently there is the problem that the optical fiber production apparatus is enlarged in size, and thus the apparatus production cost becomes high. Also, this method suffers from the problem that the dimension of the cooling apparatus in a direction orthogonal to the running direction of the optical fiber, that is, the dimension in the diameter direction of the cooling apparatus, becomes large. Further, in this method, a means of releasing the cooling air after the cooling to the outside of the cooling apparatus is needed. When the cooling air is released to the heating furnace side above the cooling apparatus, the cooling air enters into the heating furnace, resulting in a problem that the quality of the optical fiber is lowered. A detailed explanation of the problem of the cooling air entering into the heating furnace and the quality of the optical fiber being lowered will be given later as another conventional example using the figures.
JPP 60(1985)-65747 discloses as a method of force-cooling the optical fiber after the drawing in the heating furnace a method of generating a vortex flow of the cooling medium on the outer circumference of the optical fiber by using the cooling medium, cooling the optical fiber by the vortex flow of this cooling medium, and, at the same time, holding the running optical fiber by this vortex flow. In this method too, however, there is a problem that the cooling medium enters into the heating furnace above the cooling unit, to lower the quality of the optical fiber in the heating furnace. Further, in this method, there is a problem that the construction for generating the vortex flow is complex, and the construction of the cooling apparatus becomes complex.
JPP 1(1990)-208345 and JPP 2(1991)-188451 disclose a method of providing a gas cooling tube, spirally perforated toward the optical fiber, so as to enclose the periphery of the optical fiber drawn in the heating furnace, introducing the cooling gas to this cooling tube from the lower portion toward the upper portion, blowing the cooling gas from the holes of the cooling tube to the optical fiber, and thereby cooling the optical fiber. In this method too, however, since provision is made of the gas cooling tube spirally perforated toward the optical fiber, the construction of the cooling apparatus is complex and further there is a problem that the flow rate of the gas introduced into the gas cooling tube must be exactly controlled so as to uniformly cool the optical fiber, and so the control operation thereof becomes complex.
As mentioned above, force-cooling becomes necessary to coat the optical fiber drawn in the heating furnace by a protective resin, but such force-cooling has the problem that the quality of the optical fiber is lowered as mentioned above and as will be explained in detail below referring to the figures. A detailed explanation of another conventional optical fiber cooling method causing the problem of a lowering of the quality of the optical fiber due to the cooling gas flowing out from the cooling apparatus entering into the above-mentioned heating furnace will be given below as well.
FIG. 1 is a structural view of an optical fiber production apparatus having a cooling apparatus 4 for performing force-cooling using helium (He) gas, which has a good heat conductivity, as the cooling gas. The lower portion of the optical fiber preform 2 is heated to melt inside the heating furnace 1, and the optical fiber 3 is drawn from the heated and melted portion of the optical fiber preform 2. The obtained drawn optical fiber 3 is pulled out from the optical fiber exit 11 at the bottom and introduced into a cooling tubular body 5 of the cooling apparatus 4 provided downstream of the heating furnace 1. After the optical fiber 3 is force-cooled by the He gas in the cooling tubular body 5, the optical fiber 3 is passed through the resin coater 6 provided below the cooling apparatus 4 to coat a protective resin on the same. Further, the resin coated on the optical fiber 3 is cured by a resin curer 7, the orientation of the optical fiber is changed via a turn roll (or a turn sheave) 27, and the optical fiber coated by the resin is taken up by a takeup machine (not illustrated) provided further on. For example, where the drawing rate is 300 m/min, the He gas flows inside the cooling tubular body 5 at about 10 l/min, to force-cool the optical fiber 3. Since He gas is light, usually the He gas is injected from the cooling gas injecting port 8 at the lower portion of the cooling tubular body 5 and blown out from the optical fiber inlet 9 at the upper portion of the cooling tubular body 5.
In the optical fiber production apparatus shown in FIG. 1 using He gas as the cooling gas, since the mass of He gas is small and also the diffusion coefficient is large, even if the He gas is introduced into the cooling tubular body 5, the amount of the He gas leaked from the optical fiber inlet 9, which serves as the hole for passage of the optical fiber, at the top of the cooling tubular body 5 is large. The amount of the He gas to be introduced into the cooling tubular body 5 is about 10 l/min where the drawing rate is 300 m/min, and the concentration of the He gas inside the cooling tubular body 5 is always a concentration of 50 percent or less, and therefore there arises a problem that the air in the outside environment invades the cooling tubular body 5, the dust floating in the air comes into contact with the optical fiber 3 before the coating of the resin, scratches are formed in the optical fiber 3, or the like, so the optical fiber 3 breaks by a tensile force of an average strength of 6 kg or less.
Originally, so as to fill the He gas in the cooling tubular body 5, preferably no port for release of the He gas is provided, but to pass the optical fiber 3 through the cooling apparatus 4, it is necessary to provide the optical fiber inlet 9 and the optical fiber exit 10 having a diameter of about 10 mm at the upper portion and lower portion of the cooling tubular body 5. Where the drawing rate is 300 m/min and the He gas has a small flow rate, for example, about 10 l/min, the flow rate of the He gas blown out from the optical fiber inlet 9 is about 2 m/sec. However, a recent tendency has been to raise the drawing rate from 300 m/min to about 600 m/min, which is the twice the former, so as to enhance the productivity of the optical fiber. In that case, to enhance the cooling capability of the optical fiber 3, it is necessary to pass also He gas in the cooling tubular body 5 at a rate of about two times, for example 20 l/min, and He gas at a high speed of about 4 m/sec is blown from the optical fiber inlet 9 directly above the cooling tubular body 5. Where a large amount of He gas is blown out from the optical fiber inlet 9 to the area above it, the cooling He gas enters from the optical fiber exit 11 of the heating furnace 1, positioned above the optical fiber inlet 9, into the heating furnace 1 while the high speed He gas flow entrains the air at the periphery. An inert gas is filled in the heating furnace 1, but there arises a problem that the dust in the air invades the heating furnace 1, or the heating furnace material is burned by the entering gas to generate dust, and that dust adhers to the optical fiber 3 in a stage where it is formed as a core and cladding by the heating and melting inside the heating furnace 1. This damages the optical fiber 3 and lowers the strength of the optical fiber 3. That is, where the drawing rate is doubled in this way, the amount of supply of the He gas to the cooling tubular body 5, i.e., 10 l/min also doubles, and also the entraining flow of the air becomes about double, and therefore the probability of adhesion of dust to the optical fiber 3 doubles and the probability of breakage of the optical fiber 3 becomes two times or more.
FIG. 2 is a structural view of an optical fiber production apparatus for performing the drawing of an optical fiber accompanied with the conventional force-cooling using air as the cooling medium. This optical fiber production apparatus uses an inexpensive gas in place of the expensive He gas as the cooling medium, and therefore has an advantage in view of price in comparison with the optical fiber production apparatus shown in FIG. 1.
In FIG. 2, a cooling apparatus &A is provided between a heating furnace 1 and a resin coater 6, gas blowing nozzles 12a to 12d each having a structure illustrated in FIG. 3 are arranged in a plurality of stages inside the cooling tubular body 5A of this cooling apparatus 4A, and the cooling air after cleaned of dust is blown out from these gas blowing nozzles 12a to 12d to the optical fiber 3 at a high speed, to force-cool the optical fiber 3. That is, the gas blowing nozzles 12a to 12d are arranged in a plurality of stages vertically along the passage of the optical fiber 3 inside the cooling tubular body 5A of the cooling apparatus 4A. Clean air is blown out from these cooling gas blowing nozzles as the cooling gas to the optical fiber 3 at a high speed to cool the same. Among the plurality of stages of gas blowing nozzles 12a to 12d, the gas blowing nozzle 12a at the topmost stage blows out the cooling gas downward inside the cooling tubular body 5A. Note that, in the cooling tubular body 5A, other than the optical fiber 3 being cooled by the gas blowing nozzles 12a to 12d, clean air flows in from the gas feed ports 14 on the left and right in the upper portion of the optical tubular body 5A via a filter 13, to prevent the adhesion of the dust to the optical fiber 3.
Also, in an optical fiber production apparatus having a cooling apparatus 4A shown in FIG. 2, an optical fiber inlet 9 and an optical fiber exit 10 are provided at the top portion and bottom portion of the cooling tubular body 5A so as to pass the optical fiber 3, and therefore the air is blown out directly upward from the optical fiber inlet 9 at a high speed and goes toward the heating furnace 1. If the gas blowing nozzle 12a at the topmost stage is oriented upward inverse from the illustration, a larger amount of air is naturally blown out from the optical fiber inlet 9 to the heating furnace i just above the same and enters into the heating furnace i via the optical fiber exit 11 of the heating furnace. For example, when clean air is introduced from the gas feed port 14 to the inside of the cooling tubular body 5A via the filter 13 at a rate of about 2 m.sup.3 /min, the differential pressure between the inside and outside of the cooling tubular body 5A becomes about 1 mmH.sub.2 O, and therefore if the inner diameter of the optical fiber inlet 9 is about 10 mm, the air is blown out from the optical fiber inlet 9 at about 20 l/min. The gas flow rate of the air flowing out from the optical fiber inlet 9 at this time becomes about 4 m/sec. Also this air entrains the air containing dust at the outside of the heating furnace 1, enters into the heating furnace i from the optical fiber exit 11, and induces the same problems as mentioned referring to FIG. 1.
That is, in both of the optical fiber production apparatus illustrated in FIG. 1 and the optical fiber production apparatus illustrated in FIG. 2, when the drawing rate of the optical fiber 3 is set to 300 m/min or more, for example, about 600 m/min, so as to improve the productivity of the optical fiber, the cooling ability by the cooling gas must be raised so as to cool the optical fiber 3 in a shorter time. For this purpose, it is necessary to make the distance between the heating furnace 1 and the cooling apparatus 4 shown in FIG. 1 and the distance between the heating furnace i and the cooling apparatus 4A shown in FIG. 2 as short as possible, to ensure a long cooling section. A shorter cooling interval is convenient in terms of the reduction of size of the optical fiber production apparatus. However, when the cooling interval is made short, the flow rate of the cooling gas which is exhausted from the cooling apparatus 4 or the cooling apparatus 4A and rises toward the heating furnace 1, that is, the He gas or air, is not lowered that much and entrains the air containing dust at the periphery. This cooling gas enters into the heating furnace 1, to cause a problem of deterioration of the strength of the optical fiber 3 as mentioned previously.
With respect to this, as shown in FIG. 4, for example, in the optical fiber production apparatus illustrated in FIG. 2, a proposal has been made that a rise suppression gas be blown out from the optical fiber inlet 9 of the cooling tubular body 5A downward into the cooling tubular body 5A to suppress the cooling gas which is blown out from the optical fiber inlet 9 at the upper portion of the cooling tubular body 5A and rises toward the heating furnace 1. However, such a method involves a problem that it is necessary to blow downward the rise suppression gas in an amount approximately the same as that of the cooling gas from the optical fiber inlet 9 of the cooling apparatus 4A and therefore useless gas consumption occurs.
Note that, in the optical fiber production method of the optical fiber production apparatus shown in FIG. 2 using air as the cooling gas, the air has a lower cooling capability than He gas and therefore a higher speed flow of cooling air flow is formed compared with the He gas. This also entrains the air at the periphery of the optical fiber 3 inside the cooling tubular body 5A when performing the cooling, and thus the amount of entrainment of the air containing dust of the surroundings after flowing out from the optical fiber inlet 9 becomes larger. This air containing dust enters into the heating furnace 1, resulting in a problem that the probability of breakage of the optical fiber 3 due to the contact with dust becomes larger. For example, where the flow rate of the cooling air is about 20 m/sec, the probability of contact of the dust becomes 5 times or more greater in comparison with the case where He gas flows.
Where cooling air is blown out to the periphery of the optical fiber 3 to cool the same, the level of dust of the environment (the periphery of the cooling apparatus) and the probability of the breakage of the optical fiber 3, that is, the number of times of breakage per unit length where 1 percent elongation is applied to the optical fiber 3 by screening has a correlative relationship as shown in, for example, FIG. 5. Where a long length, for example, 100 km or more, of an optical fiber 3 is to be produced as in recent years, the probability of breakage of the optical fiber 3 must be reduced to 0.01 break/km or less. For this purpose, an environment of a degree of cleanness of class 1000 or better is needed, and the manufacturing cost of the cooling apparatus and consequently the manufacturing cost of the optical fiber production apparatus becomes higher.
In this regard, it has also been considered to place the entire optical fiber production apparatus in a clean environment, but an optical fiber production apparatus has a total length of a long as 10 m or more, and the facility cost becomes further higher when placing an entire optical fiber production apparatus in a clean environment. Also, the running cost of the optical fiber production apparatus becomes higher. Therefore, this is not practical.